This is an application for a K23 award for Dr. Anne Marie Singh, a pediatric allergist-immunologist at Northwestern Feinberg School of Medicine. Dr. Singh is young investigator in patient-oriented translational research of food allergy. Her previous research studying the phenotypic expression of early eczema and role of regulatory T cells in allergy makes her uniquely qualified to study early mechanisms of loss of tolerance. Food allergy affects over 12 million Americans, and prevalence in children has increased by 18% from 1997-2007. Food allergy-induced anaphylaxis is often life threatening, and food allergy is an important risk factor for the future development of asthma and allergic rhinitis. Despite its significant impact, little is known regarding the cellular and immunologic mechanisms in the initiation of food allergy in children. Food allergy results from a loss of oral tolerance t ingested antigens, leading to the pathologic immune response. Regulatory T (Treg) cells are important in balancing immune responses, and dysregulation of these cells may play an important role in the pathogenesis of food allergy. It has recently been shown that oral exposure to Staphylcoccal enterotoxin B, SEB, breaks oral tolerance in mice, promoting food allergy-associated responses. Exciting new preliminary data demonstrate increased Staphylococcus aureus burden in the gut of patients with food allergy, and previous work has also illustrated increased IL-10 producing Tregs in formally egg allergic children as they acquire tolerance. It has been shown that topical S. aureus plays a crucial role in the cutaneous inflammation of atopic eczema and activation of CD25+ Treg cells with S. aureus enterotoxin B (SEB) abrogated their suppressive phenotype. Together, these data support the following hypothesis: colonization of children with Staphylococcus aureus abrogates Treg function, inhibiting oral tolerance, and thereby promotes allergic inflammation and the development of food allergy. To test this hypothesis we propose to characterize the Treg cells by flow cytometry in children with food allergy compared to non-food allergic children, determine the colonization and enterotoxin production of S. aureus in food allergic and non-food allergic children through microbiome analysis, and determine the effect of food antigen and microbial exposure on Treg function in vitro in these children. This proposal focuses on the role of Treg cells and S. aureus in the attendant loss of tolerance, and will open approaches to prevent the development of food allergy or increase treatment options beyond antigen avoidance. Through the proposed research, Dr. Singh will work towards her long-term goal of becoming an independent translational physician-investigator, studying cellular mechanisms of early life exposures on the development of atopy. Short term goals are to: (1) develop research skills of regulatory T cell analysis, transcription factor analysis and next-generation sequencing, (2) establish bioinformatics skills to analyze microbiota data sets and (3) develop biostatistical and clinical trial skills for study design and implementation. After completion of this award, Dr. Singh, will have had the training to design and implement studies on the immune effects of bacterial and environment exposures, including the acquisition of microbiota over time and the mechanisms that microbial and other environmental exposures impact the regulation of the immune response. To achieve these goals, a top-notch mentoring team has been assembled. The primary mentor, Robert Schleimer, PhD, Chief, Division of Allergy-Immunology (Department of Medicine) has a strong track record of translational research in allergy and has successfully trained many academicians. Co-mentors include Paul J. Bryce, PhD, Associate Professor, with expertise in mouse models of food allergy and regulatory T cell studies~ Jacqueline Pongracic, MD, Chief, Division of Allergy-Immunology (Department of Pediatrics) who has multicenter clinical trial experience in pediatric allergy patients~ Lewis Smith, MD, Associate Vice Chair for Research, with extensive clinical and laboratory-based research programs studying asthma pathogenesis~ and Mitchell L. Sogin, PhD, Director, Marine Biology Laboratory, a pioneer in studies of bacterial communities, including bioinformatics analysis of human microbiota. The mentoring team will oversee Dr. Singh's progress to study the effects of S. aureus on regulatory T cell function in food allergy.